Thin film solar cells on flexible substrates are today limited to amorphous silicon on a thin metal foil (usually stainless steel) and copper indium gallium diselenide (CIGS) on metal or polyimide foils. Undoubtedly, there will be other types of solar cell materials suitable for use on flexible substrates available at some time in the future. Thin film cadmium telluride solar cells are currently produced only on glass. To be useful in a solar power system, any type of solar cell must be electrically interconnected serially with other similar solar cells to raise the voltage levels and minimize I2R losses due to high currents. Cells deposited on rigid sheets of glass generally use a system of scribes applied between different process steps and at specific locations to interconnect the cells over the entire sheet. This procedure is called “monolithic integration”. Such a method is difficult to implement on flexible substrates because of the accuracy required for both the placement and depth of the scribes. Additionally, flexible substrates enable roll-to-roll processing which can become less desirable economically if the process is interrupted to implement the scribing operations even if they could be readily done.
Traditional crystalline or polycrystalline silicon solar cells are formed on individual wafers, which then must be interconnected. Current collecting grids and buss bars are usually formed by screen-printing a pattern with silver bearing inks that are subsequently cured at high temperatures (on the order of 700° C.). The traditional grid pattern consists of a series of fine straight and parallel lines spaced a few millimeters apart with two or three wider lines (buss bars) running perpendicular to the pattern of fine lines. The resulting structure provides a surface on the buss bars to which interconnecting tabs can be attached by conventional soldering methods. The cell current is collected by the relatively narrow grids and transmitted to the relatively wider buss bars which then become the connection points to the next cell. An advantage of this method over monolithic integration is that the cells can be tested and sorted for performance prior to module build. In this way the module performance is not limited by the lowest performing cell in the string.
The same method applied to thin film flexible solar cells has met with only limited success. At least two problems are generally encountered. First, the thin film cells cannot survive the high temperatures needed to adequately cure the silver inks. As a result of lower curing temperatures some of the ink carriers and solvents remain in the grid line structure, which lowers the conductivity and severely limits the solderability of the printed buss bars. Alternatively, the interconnection may be made with conductive epoxies, but the method is mechanically and electrically inferior to soldering. Secondly, since the surface finish of useful flexible substrates is much rougher than that of glass or silicon wafers, many more defects exist which can become shunt sites if conductive ink is allowed to flow into them. This problem can be somewhat mitigated by first printing a much less conductive material, like a carbon ink to initially fill the defects, and then over printing with the silver ink. Consistently good results are very difficult to achieve, since anything short of perfect registration causes extra shading loss as well as increased potential shunting. In addition the cost of the materials and equipment is relatively high.
U.S. Pat. No. 5,474,622, which is entirely incorporated herein by reference, teaches using metallic wires as grids, but with the wires coated with carbon fibers of sufficient length to avoid being forced into defects. In this method the wires were attached to the top electrode (transparent conductive oxide or TCO) of the thin film amorphous silicon solar cells during the process of laminating them into modules. In effect the previous art of printing a carbon based ink pattern first is replaced with carbon fibers that have much less chance of causing shunts in film/substrate defects and at the same time provide a fusing type of protection against sustained heavy shunt currents. The wire size and spacing must be selected so as to carry the current generated by the cell without generating significant resistive losses.
U.S. Pat. Nos. 4,260,429 and 4,283,591, which are entirely incorporated herein by reference, teach coating conductive wires with a polymer that contained conducting particles. Problems with defect-induced shunts could still exist because of smaller conductive particles in the distribution, and improvements were put forth in U.S. Pat. No. 6,472,594, which is entirely incorporated herein by reference.
Regardless of the detailed way in which the possible shunt paths are dealt with when applying the conductive grids to flexible solar cells, no comprehensive, automated, and economical interconnection scheme has been developed for flexible solar cells which possesses many of the automated features of monolithic integration on rigid glass. It is the purpose of the present invention to present an improved interconnect scheme which might be referred to as “pseudo monolithic integration” suitable for automated implementation for flexible solar cells.